In general, an anti-lock control apparatus for a vehicle is performed by a control unit including a microcomputer so that hold valves and decay valves are operated to open or close on the basis of electrical signals representing wheel speeds detected by wheel speed sensors, thereby increasing, holding or decreasing the brake hydraulic pressure, for the purpose of securing improved steering performance, running stability, and reducing the braking distance of the vehicle during the braking operation of the vehicle.
FIG. 1 shows a conventional control state diagram as disclosed in U.S. Pat. No. 4,741,580 which illustrates changes in the wheel speed Vw, wheel acceleration and deceleration value dVw/dt and brake hydraulic pressure Pw, as well as corresponding hold signal HS and a decay signal DS for opening and closing hold valves and decay valves.
When no braking operation is performed while the vehicle is running, the brake hydraulic pressure Pw is not increased and both the hold signal HS and the decay signal DS are in the off-state, so that the hold valve opens whereas the decay valve closes. With a brake operation, on the other hand, the brake hydraulic pressure Pw increases rapidly from time point t0 (normal mode), thereby reducing the wheel speed Vw. There is provided a reference wheel speed Vr which is lower by a predetermined amount .DELTA.V than the wheel speed Vw and follows the latter with that speed difference. The reference wheel speed Vr is so determined that when the wheel deceleration (negative acceleration) value dVw/dt reaches a predetermined threshold value, -1 G, for instance, at a time point t1, it decreases linearly from the time point t1 with a deceleration slope .theta. of -1 G.
At a time point t2 when the wheel deceleration value dVw/dt reaches a predetermined value -Gmax (-2 G, for example) with maximum absolute value, the hold valve is closed by turning on the hold signal HS to hold the brake hydraulic pressure Pw.
With holding the brake hydraulic pressure Pw, the wheel speed Vw further decreases to be less than the reference wheel speed Vr at a time point t3. At the time point t3 the decay signal DS is turned on to open the decay valve to thereby start reducing the brake hydraulic pressure Pw. As a result of the pressure reduction, the wheel speed Vw changes from its decrease to increase at a time point t4 at which the wheel speed Vw is in a low peak Vl condition. At the time point t4, the decay signal DS is turned off to close the decay valve, so that the reduction of the brake hydraulic pressure Pw is terminated and the brake hydraulic pressure Pw is kept being the same.
Next, when the wheel speed Vw attains a high peak Vh at a time point t5, the brake hydraulic pressure Pw increases again. The increase of the brake hydraulic pressure Pw in this stage is arranged to take place gradually by a repetition of turning on and off the hold signal HS mincingly, thereby reducing in the wheel speed Vw calmly. Starting at a time point t6 (corresponding to t3) the decompression mode is carried out again.
On the other hand, the estimated vehicle speed Vv is calculated as a speed close to the actual vehicle speed. The estimated vehicle speed Vv is calculated by selecting the highest wheel speed VwH among the four vehicle wheels (select-high), and then defining a followable limit of the speed to the selected highest wheel speed VwH within a predetermined range (.+-.1 G, for example) in the vehicle acceleration and deceleration conditions sides. A rate of the opening (OFF) and closing (ON) times of the hold valve after the time point t5 shown in FIG. 1, that is, an acceleration rate of the hydraulic pressure, is determined on the basis of a frictional coefficient .mu. of the road surface judged in accordance with the deceleration value of the estimated vehicle speed Vv between the time points t1 and t5. Further, not only the acceleration rate but also some parameters such as the value of .DELTA.V and the termination point of the pressure decrease are varied by judging the frictional coefficient .mu. of the road surface in accordance with an inclination of the deceleration of the estimated vehicle speed Vv from the acceleration starting point to the next acceleration starting point in the anti-lock control cycle after the time point t6.
Generally, when the above-described anti-lock control is actually applied to vehicles, a three-system anti-lock control method is widely used, in which method each of the front wheels of the vehicle is controlled independently on the basis of the respective wheel speeds (a first and second system speeds), while the rear wheels thereof are controlled on the basis of the lower one (a third system speed) of the rear wheel speeds (select-low). The frictional coefficient .mu. of the road surface is judged in every control systems in accordance with the inclination of deceleration of the estimated vehicle speed Vv between the hydraulic pressure starting point and the next hydraulic pressure starting point.
However, the above-described conventional control method would suffer from the following problems.
As shown in FIG. 2, comparing the wheel speeds of the front-left and front-right wheels, both wheel speeds may not be synchronized at all the time. In this case, one of the hydraulic pressure starting point of the system speed would be different from the other in the two control systems. On the other hand, since the estimated vehicle speed Vv does not decelerate with a constant inclination, even if the frictional coefficient of the road surface is constant, an inclination of deceleration .DELTA.v1/.DELTA.t1 between the hydraulic pressure starting point of the front-right wheel to the next starting point thereof during the time period .DELTA.t1 would be different from that .DELTA.v2/.DELTA.t2 between the hydraulic pressure starting point of the front-left wheel to the next starting point thereof during the time period .DELTA.t2. Accordingly, there may occur a case in which a judgement of a frictional coefficient .mu. of the road surface is different between the control systems for the front-left wheel and front-right wheel. Thus, the conventional anti-lock control method would not be stable.